If you used Sign in with Apple, the email attached to your Chatex account may end in @privaterelay.appleid.com.
In edge cases, the account can also end up with a relay-like fallback address, so the exact stored address may not always appear in Apple settings.
What to do
Try Apple sign-in again with the same Apple account before creating a new Chatex account.
If you are recovering a paid account, include any Apple relay address you know, your support code, and any charge details in the support flow.
Apple also has its own help documentation for Hide My Email and Sign in with Apple if you want to confirm how Apple relay addresses work.